In reflection densitometry there is often a requirement to illuminate a sample at 45.degree. with light of a narrow beam width, see International Standards Organization ISO Standard 5/4 "Geometric Conditions for Reflection Density". This ideally takes place over a full 360.degree. cone to ensure that the orientation of directionally reflective materials does not effect the resulting measurement.
If an optical system for forming a narrow annular beam converging at 45.degree. onto a spot on a sample to illuminate that spot is formed substantially as is described in GB-A-2191572 or, alternatively, comprises a lamp mounted at the center of the reflective surface of a paraboloidal reflector so as to produce a substantially parallel beam of light, a disc mounted co-axially with the axis of the paraboloidal reflector in the path of the parallel beam of light so as to restrict that beam such that the light that passes it is an annular, parallel beam of light co-axial with the axis of the reflector, and a simple verging lens mounted co-axially with the axis of the paraboloidal reflector on the other side of the disc remote from the paraboloidal reflector whereby the annular parallel beam is caused to converge and focus on the spot which is on the axis of the paraboloidal reflector, the required lens must have a large focal ration (f/no). This is difficult to achieve and, in addition, there is little space available between the lens and the sample to house the necessary detector which senses light reflected by the sample perpendicular to its plane.